The invention has been developed primarily for use with the treatment of organically polluted aqueous phases, especially oily wastewater and sewerage and will be described hereinafter with reference to this application. However, it will be appreciated that the invention is not limited to this particular field of use and could be used for the treatment of other types of aqueous waste streams such as leachate, run-off, and those streams containing oxidizable inorganic matter.
Hitherto, the secondary treatment of wastewater has generally been achieved through the activated sludge process or other like methods. Due to the large variation in both the flow and content of the wastewater to be treated, the different stages of the process have been carried out in a relatively independent way. As a result a substantial excess of material is left untreated, particularly during peak flows. This material commonly remains untreated. A further effect is that the variations are carried through to the quantities of biomass produced by such treatment, commonly called "sludge" as well as primary solids. For purposes of the present invention, "sludge" will include primary solids.
This problem is further exacerbated due to the wastewater treatment plants being generally designed for maximum capacity. When the supply of wastewater increases due to diurnal, seasonal and special factors, the plant may become overloaded due to the subsequent increase in wastewater volume or pollutant concentrations. It is generally understood that the limiting factor for pollutant treatment in such a system is the oxygen transfer capacity of the aeration system, since oxygen exposure times are set by the treatment plant hydraulic characteristics, such characteristics being influenced solely by flow, irrespective of the requirement of the pollutants for oxygen for effective treatment.
Furthermore, in certain circumstances, to ameliorate the overloading problem of the treatment system due to extra inflows of wastewater, large storage tanks or "lagoons" are provided. However, in an urban environment where space is at a premium, this alternative is not always available.
Previously, new specially designed tanks have been used to reduce the retention time when oxygen has been used for aeration. However, as mentioned above, this is often a relatively expensive alternative. Conversion of existing tanks is, at best, difficult and costly.
To overcome the limits on aeration capacity, it is possible to provide supplemental oxygen to the wastewater by such means as disclosed in U.S. Pat. No. 4,163,712. This technique requires that oxygen generated at a remote location be transported to the plant resulting in considerable costs associated with liquid production, storage and transportation.
Alternatively, on-site generation of oxygen has also been used in hope of reducing the above mentioned costs. However, the high capital costs of methods of producing the oxygen from air require machinery which is preferably operated at close to the maximum design capacity. This feature of such machinery is not well suited to provision of oxygen due to the large diurnal and seasonal fluctuation of the wastewater flow and composition. Although such on-site methods have been economically impractical to implement, effective pollution control can be established.
Furthermore, when air not enriched with oxygen is used as a source of oxygen for such wastewater treatment, the partial pressure of nitrogen is much greater than the partial pressure of oxygen. Consequently, only a relatively small amount of oxygen is absorbed into solution and excessive frothing of the wastewater may occur. In contrast, when substantially pure oxygen is used in an efficient manner, then poor stripping of carbon dioxide occurs, causing excessive acidity.